bug#39948: 28.0.50; crash in fchmodat

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Date: Fri, 06 Mar 2020 15:16:43 +0100
> 
> I updated from master today and now Emacs is crashing when I use Gnus.
> The first time it happened I been reading news groups for a while, then
> email arrived and when I pulled it into Gnus, Emacs crashed.  Then I
> restarted Emacs under GDB and now get the crash already on starting Gnus
> (with my initializations; it doesn't happen when I start an unconfigured
> Gnus in Emacs -Q).  I tried to get a full backtrace, but the output of
> `bt full' seemed to be in an endless loop; here's the start of the
> backtrace:
> 
> Thread 1 "emacs" received signal SIGSEGV, Segmentation fault.
> 0x000000000060c9b2 in fchmodat (dir=dir@entry=-100,
>     file=file@entry=0x4be6b40 "/home/steve/Mail/unsorted.nov",
>     mode=mode@entry=384, flags=flags@entry=0)
>     at /home/steve/src/emacs/emacs-master/lib/fchmodat.c:65
> 65      {
> (gdb) bt full
> #0  0x000000000060c9b2 in fchmodat
>     (dir=dir@entry=-100, file=file@entry=0x4be6b40 
> "/home/steve/Mail/unsorted.nov", mode=mode@entry=384, flags=flags@entry=0)
>     at /home/steve/src/emacs/emacs-master/lib/fchmodat.c:65
> #1  0x000000000060cae4 in orig_fchmodat
>     (dir=dir@entry=-100, file=file@entry=0x4be6b40 
> "/home/steve/Mail/unsorted.nov", mode=mode@entry=384, flags=flags@entry=0)
>     at /home/steve/src/emacs/emacs-master/lib/fchmodat.c:33
> #2  0x000000000060c9e0 in fchmodat
>     (dir=dir@entry=-100, file=file@entry=0x4be6b40 
> "/home/steve/Mail/unsorted.nov", mode=mode@entry=384, flags=flags@entry=0)
>     at /home/steve/src/emacs/emacs-master/lib/fchmodat.c:134
> #3  0x000000000060cae4 in orig_fchmodat
>     (dir=dir@entry=-100, file=file@entry=0x4be6b40 
> "/home/steve/Mail/unsorted.nov", mode=mode@entry=384, flags=flags@entry=0)
>     at /home/steve/src/emacs/emacs-master/lib/fchmodat.c:33
> #4  0x000000000060c9e0 in fchmodat
>     (dir=dir@entry=-100, file=file@entry=0x4be6b40 
> "/home/steve/Mail/unsorted.nov", mode=mode@entry=384, flags=flags@entry=0)
>     at /home/steve/src/emacs/emacs-master/lib/fchmodat.c:134
> #5  0x000000000060cae4 in orig_fchmodat
>     (dir=dir@entry=-100, file=file@entry=0x4be6b40 
> "/home/steve/Mail/unsorted.nov", mode=mode@entry=384, flags=flags@entry=0)
>    ter/lib/fchmodat.c:33
> 
> This pattern repeated for tens of thousands of frames, then I
> interrupted it and typed `c':

Sounds like infinite recursion, which causes stack overflow.